If you're running a UPS on "house battery power", you obviously have an inverter. Why not just run the computers on the inverter all the time? If you're connected to shore power, the converter/charger will power the inverter, and if it cuts out, the house batteries take over the load. Putting another battery and charger and inverter in between the one you have and your load seems rather counterproductive to me.
I suspect UPSs are not used for the balanced/isolated power supply in a studio mainly because the power most put out (when running off their batteries) is none too clean and would cause all sorts of audio noise. Most UPSs, at least most relatively inexpensive ones, are MSW output—fine for computers with switching power supplies, but inducing noise in audio signals.
